Transport Across Membranes

Functions of Membranes 1. Protect cell 2. Control incoming and outgoing substances 3. Maintain ion concentrations of various substances 4. Selectively permeable - allows some molecules in, others are kept out

Fluid Mosaic Model

Phospholipid Bilayer

The main macromolecules in membranes are lipids and proteins, but include some carbohydrates. The most abundant lipids are phospholipids. Phospholipids and most other membrane constituents are amphipathic molecules. Amphipathic molecules have both hydrophobic regions and hydrophilic regions.

Membranes are fluid Membrane molecules are held in place by relatively weak hydrophobic interactions. Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other. Protein drift is more restricted. Cytoskeleton, motor molecules.

Membrane fluidity is influenced by temperature and by its constituents. Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails prevent tight packing. Cholesterol. Temperature and adaptive changes in lipid composition.

Methods of Transport Across Membranes 1. Diffusion -passive transport - no energy expended 2. Osmosis - Passive transport of water across membrane 3. Facilitated Diffusion - Use of proteins to carry polar molecules or ions across 4. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP

Diffusion Movement of molecules from an area of high concentration to an area of low concentration. Movement from one side of a membrane to another, un-facilitated

Diffusion

Movements of individual molecules are random. Diffusion is the tendency of molecules of any substance to spread out in the available space Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules. Movements of individual molecules are random. However, movement of a population of molecules may be directional. Concentration gradient.

The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen. The concentration gradient represents potential energy and drives diffusion. However, because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate. Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water.

Solutions Solutions are made of solute and a solvent Solvent - the liquid into which the solute is poured and dissolved. We will use water as our solvent today. Solute - substance that is dissolved or put into the solvent. Salt and sucrose are solutes.

Osmosis is the passive transport of water Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. The solution with the higher concentration of solutes is hypertonic. The solution with the lower concentration of solutes is hypotonic. These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water. Solutions with equal solute concentrations are isotonic. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Tonicity is a relative term Hypotonic Solution - One solution has a lower concentration of solute than another. Hypertonic Solution - one solution has a higher concentration of solute than another. Isotonic Solution - both solutions have same concentrations of solute.

Osmosis

Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer. This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. Osmosis continues until the solutions are isotonic.

The direction of osmosis is determined only by a difference in total solute concentration. The kinds of solutes in the solutions do not matter. This makes sense because the total solute concentration is an indicator of the abundance of bound water molecules (and therefore of free water molecules). When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net osmosis. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Plant and Animal Cells put into various solutions

Types of Transport

Active Transport Transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentration) with the aid of proteins in the cell membrane and energy from ATP.

Active Transport

Movement in Vesicles Endocytosis: Pinocytosis: Phagocytosis: The uptake by a cell of material from its environment by a process in which the cell surrounds the material and engulfs it with a vesicle formed by its plasma membrane. Pinocytosis: A type of endocytosis whereby soluble molecules and fluids are taken up from outside the cell through the formation of vesicles. (literally, "cell eating") is a form of endocytosis where large particles are enveloped by the cell membrane of a cell and internalized. Phagocytosis:

Movement in Vesicles Exocytosis : The release of materials in a vesicle to the outside of a cell. The opposite of endocytosis. Cells use exocytosis to release large molecules such as proteins waste products, or toxins that would damage the cell if released in the cytosol. Proteins are made on ribosomes on the Rough endoplasmic reticulum, and packaged into vesicles by the Golgi Apparatus

Exocytosis and endocytosis transport large molecules Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins. Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles. During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. When the two membranes come in contact, the bilayers fuse and spill the contents to the outside. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Endocytosis is a reversal of exocytosis. During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane. A small area of the palsma membrane sinks inward to form a pocket As the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

One type of endocytosis is phagocytosis, “cellular eating”. In pinocytosis, “cellular drinking”, a cell creates a vesicle around a droplet of extracellular fluid. This is a non-specific process. Receptor-mediated endocytosis is very specific in what substances are being transported.